Antitarnish



ANTITARNisrr APPERS Daniel Gray, Sherrill, N. Y., assignor to OneidaLtd, Oneida, N. Y., a corporation of New York No Drawing. ApplicationDecember 4, 1951, Serial No. 259,997

14 Claims. (Cl. 21-25) This invention relates to protecting silveragainst tar- 'nishin'g and more particularly to cloth, paper, or othersuitable covering or Wrapping material impregnated with sulfur-reactingchemicals and adapted to be used to wrap silverware to protect it fromtarnishing.

In the past many different chemicals have been employed 'a's impregnantsfor cloth, paper, etc. used for wrapping or storing articles of silver,including both solid silver and silverplate. All of such treated clothshave to date had disadvantages. Some of the proposed impregnants releasestrong acids upon reaction, some become dark upon exposure to sulfurcontaining compounds, some do not remain in the cloth, and others areinsufficiently reactive, 01" are too toxic for use.

it is an object of this invention to prepare a non-corroding antitarnishwrapper for articles of silver having an exceptionally highsulfur-combining property.

A further object is to prepare an antitarnish wrapper which does notalter in color after extended use.

A still further object is to prepare an anti'tarnish wrapper containingrelatively insoluble salts which are thoroughly incorporated in materialsuitable for wrapping silverware.

A still further object is to prepare an antitarnish wrapper having ahigh stoichiometric ratio of impregnant to sulfur.

These objects may be accomplished by impregnating a wrapping materialwith an aqueous solution of a zinc salt of .a weak acid such as zincacetate. The excess solution is removed 'andthe zinc is precipitated asa zinc carbonate or a zinc borate by means of a solution of sodiumcarbonate or sodium borate, the excess alkali salt squeezed out, thewrapping material dried, and any excess powder is removed.

The zinc salt may be used in any desired concentration, but I have foundthat 8 to 16 ounces of salt for each gallon of water yields a verysatisfactory product. The alkali metal carbonate or borate may also bepresent in any desired concentration though I have found that from 1.5to 2.5 ounces of salt per gallon of water is preferred. It is evidentthat the concentration of the salts in the solutions governs the finalweight of metal precipitated .in the wrapping material. I have foundthat from 0.5 to by weight of zinc on the total weight of material isrequired, and I prefer to precipitate from 1% to 3.5% zinc by weight.The solution concentrations given above will yield my preferred degreeof deposition of zinc when excess of the solutions is removed. It is tobe understood that when Wrappers are to be constantly exposed todeleterious gases, higher concentrations are required for long life thanwhen the silver is semiprotected as is the case when it is in a chest.If the amount of zinc is below 0.5% by weight, the protection is oftennot suflicient for long periods of time; if the amount of zinc exceeds5%, difficulty with dusting is encountered.

In the above description the order of treatment consists in firstimpregnating the wrapper with the solution of the zinc salt of a-weakacid, followed by the alkali dip. This is the preferred order. If thereverse order is carried out, i. e. the wrapper first immersed in thealkali carbonate or borate, followed by immersion in the solution of thezinc salt, the wrapper contains much less zinc carbonate or zinc boratethan when "treated by the preferred method. The reason for this isbelieved to be due to incomplete precipitation and therefore lack ofproper concentration of insoluble zinc in the interior of the wrapper.In any case it has been ascertained that the order of treatment isimportant to obtain maximum. effectiveness.

The effect of such a treatment is to impregnate the: fabric with arelatively insoluble salt of zinc, the anion of which is very weaklyacid, together with a small amount of residual sodium carbonate orsodium borate and sodium acetate. Upon attack of the zinc compound by asulfur-containing compound such as H28, the sulfide of zinc is formedtogether with an innocuous acid. The acid is immediately neutralized bythe basic sodium salt of the weak acid present in the wrapper. If thefabric has been impregnated with zinc acetate followed by sodiumcarbonate, 2. carbonate is formed; if the Wrapper has been impregnatedwith zinc acetate followed by sodium borate, a borate is formed. Thecarbonate or borate may or may not be the normal carbonate or borate butis believed to be a basic compound. In any case, the compound form'edreacts readily with hydrogen sulfide to form a white zinc sulfide saltwhich will not alter the color of the cloth. Carbon dioxide formed willescape without damage to the silver present and boric acid is notdamaging. While sodium bicarbonate may be used in place of sodiumcarbonate, it is not preferred.

Both zinc carbonate and zinc borate are white and relatively insoluble.Thus the wrapping material containing either of these salts may bewashed without substantial loss of antitarnish activity, although the pHof the wrapping material may drop below the optimum point if thematerial is washed frequently, which is not ordinarily the case. Thewrapping material may be any one of the usual materials adapted for thepurpose of wrapping silverware.

The pH of the resulting salts impregnated in the cloth should be inexcess of 7 and below about 10. It has been found that HzS reacts mostreadily with this salt mixture when the pH of the latter is betweenabout 7.5 and 9.0 and as such a pH range is safe for both the wrappingmaterial and the enclosed silver, and may be readily obtained byadjusting the amount of the residual sodium carbonate (or borate) andsodium acetate in the wrapper. It is my preferred range.

While other salts of zinc than zinc acetate may be employed in the firstbath, it is important that they be salts of weak acids which do notreact appreciably with silver, for otherwise the salt formed from thereaction in the second bath will not be alkaline and hence will not beuseful .in adjusting the pH of the salt mixture in the wrapper.

Cadmium salts as impregnants are to be avoided because of theirrelatively high toxicity.

A most surprising result of my invention is that the zinc present as'zinc carbonate or zinc borate has a sulfur combining power in the orderof three times that of the corresponding soluble zinc salt from which itwas prepared. This high combining power is of considerable importancesince it serves to extend the life of the wrapper as well as enhancingits effectiveness during its life.

In order to illustrate my invention the following examples are given:

Example I Example II The following was carried out to determine theweight of sulfur which various treated cloths were capable of pickingup. Identical pieces of cloth were treated with zinc acetate and withzinc acetate and sodium carbonate. The procedure described in Example Iwas followed except that one of the zinc acetate pieces was not dippedin sodium carbonate.

Treated Treated with zinc Untreated with zinc acetate acetate and sodiumcarbonate Percentage by weight of zinc after treatment 5. 0 3. 1Percentage of sulfur by weight picked up by exposure to hydrogensulfide. 0.05 3. 06 5. 50

It is evident that although there was less zinc present in the clothtreated in accordance with my invention than in the zinc acetate treatedmaterial, the amount of sulfur picked up was considerably in excess ofthat picked up by the zinc acetate treated material.

Example III Pieces of gray double napped cotton duvetyn heavyweightcotton cloth were treated in the following manner:

One piece was used as a control. A second piece was dipped in andthoroughly wet in zinc acetate solution containing 12 ounces per gallon,crushed as near to dryness as possible and air dried. A third piece wasthoroughly wet in the zinc acetate solution, the excess solution wrungout, then dipped in sodium tetraborate, pentahydrate solution containing9.2 ounces per gallon, crushed as near to dryness and dried.

The pH of the control was 7.7, the zinc acetate treated material 6.7 andthe zinc acetate-sodium tetraborate material 8.75. The three pieces wereplaced in a desiccator r containing an atmosphere saturated withhydrogen sulfide. The cloth remained in the desiccator hours. The pH ofthe untreated cloth was 4.8, the zinc acetate treated material 4.6, andthe zinc acetate-sodium tetraborate material 8.0. It is evident that thematerial made in accordance with my invention stays alkaline whileuntreated and zinc acetate treated cloth becomes acid upon exposure toHzS.

Silver articles Wrapped in cloth treated in accordance with my inventionas described in this example are preserved for long periods of timeagainst tarnish.

While I do not wish to be bound by my theory of the reasons for theparticular usefulness of the zinc salts of this invention, the followingis given as an explanation.

Zinc salts, as do nearly all inorganic salts, enter into a reaction inthe presence of moisture, and therefore a salt having at least somesolubility is required since an extremely insoluble compound is ratherunreactive. Zinc oxide, for instance, is reported in the Handbook ofChemistry and Physics 28th ed. to be soluble to the extent of only0.00016 gram per 100 mol. of water, and is not suitable.

Hydrated zinc acetate, on the other hand, is readily soluble in waterand may be easily removed by washing. Although the presence of moisturenormally present in 4 the air will cause sufficient ionization for areaction, hygroscopic salts are neither necessary nor desirable. Addedingredients such a glycerine serve only to cause unnecessary dampness inthe wrapper.

Thus the ideal salt is one that is readily reactive with sulfurcontaining compounds, is white, forms an insoluble white sulfide, isnon-hygroscopic, and is soluble only to a limited extent. Otherrequirements such as lack of toxic qualities, a high stoichiometricratio of sulfur to weight of compound employed and, upon reaction withhydrogen sulfide, the release of a non-injurious acid, are also ofimportance. Finally the pH of the resulting salt is important as an aidto rapid and complete reaction with sulfur compounds.

I have found that the zinc carbonates or zinc borates formed by addingan alkali metal carbonate or borate to a zinc salt of a weak acid areideal when impregnated into a wrapper or the like in the presence ofsulficient alkaline salts to raise the pH above about 7 and below 10,and preferably between 7.5 and 9.5.

The presence of some sodium carbonate or borate and sodium acetate inthe antitarnish wrapper serves to neutralize any acid gases present suchas sulfur dioxide. The life of the wrapper is thereby extended, and mademore effective, sinc zinc sulfide is relatively unstable towards strongacids and if no basic salts were present in the wrapper, active acidgases would cause zinc sulfide to react and form hydrogen sulfide.

While my invention is particularly applicable to wrapping materials suchas cotton cloth which may be laundered, other wrapping materials such astissue paper, kraft paper and boxboard may advantageously'be treated bythe process of my invention.

This is a continuation-in-part of my application Ser. No. 473 filedJanuary 3, 1948, now abandoned.

I claim:

1. An article of manufacture for protecting silverware from tarnishcomprising a fabric impregnated with a salt of zinc, the anion of whichis a member of the group consisting of a carbonate and a borate andcontaining sodium acetate, said impregnated fabric when moistened havinga pH from 7.0 to 10.0.

2. An article of manufacture according to claim 1 wherein the zinc saltis a zinc borate.

3. An article of manufacture for protecting silverware from tarnishcomprising a fabric impregnated with a tarnish-proofing compositionconsisting essentially of a salt of zinc, the anion of which is a memberof the group consisting of a carbonate and a borate, together withsodium acetate and an alkali metal salt of said ion, the pH of saidimpregnated fabric when moistened being between 7.0 and 10.0.

4. An article of manufacture for protecting a silverware from tarnishcomprising a fabric impregnated with a tarnish-proofing compositionconsisting essentially of a zinc carbonate, sodium acetate, and sodiumcarbonate, the pH of said salts when moistened being between 7.5 and9.0.

5. An article of manufacture for protecting silverware from tarnishcomprising a fabric impregnated with a tarnish-proofing compositionconsisting essentially of a zinc borate, sodium acetate, and sodiumborate, the pH of said salts when moistened being between 7.5 and 9.0.

6. A process for protecting articles having a silver surface whichcomprises impregnating a Wrapping material with an aqueous solution of azinc salt of acetic acid, precipitating a zinc carbonate in saidwrapping material by impregnating said material with an aqueous solutionof an alkali metal carbonate, in an amount in excess of that required toreact with said zinc salt and sufiicient to provide an alkalinecondition in said Wrapping material when the same is moistenedsubsequent to its manufacture, drying said wrapping material, andwrapping an article having a silver surface with said material.

7. A process for protecting articles having a silver surface whichcomprises impregnating a wrapping material with an aqueous solution of azinc salt of acetic acid, precipitating a zinc borate in said wrappingmaterial by impregnating said material with an aqueous solution of analkali metal borate, in an amount in excess of that required to reactwith said zinc salt and sufficient to provide an alkaline condition insaid wrapping material when the same is moistened subsequent to itsmanufacture, drying said wrapping material and wrapping an articlehaving a silver surface with said material.

8. A process for protecting articles having a silver surface whichcomprises impregnating a wrapping material with an aqueous solution ofzinc acetate, said zinc acetate solution having a concentration of from8 to 16 ounces of salt per gallon, precipitating a zinc carbonate insaid wrapping material by impregnating said material with an aqueoussolution of sodium carbonate, said sodium carbonate solution having aconcentration of from 1.5 to 2.5 ounces per gallon, drying said wrappingmaterial, and wrapping an article having a silver surface with saidmaterial.

9. The process in accordance with claim 8 wherein the wrapping materialis cotton cloth.

10. The process in accordance with claim 8 wherein the wrapping materialis paper.

11. A process for protecting articles having a silver surface whichcomprises impregnating a wrapping material with an aqueous solution ofzinc acetate, said zinc acetate solution having a concentration of from8 to 16 ounces of salt per gallon, precipitating a zinc borate in saidwrapping material by impregnating said material with an aqueous solutionof sodium tetraborate solution having a concentration of from 1.5 to 2.5ounces per 6 gallon, drying said wrapping material, and wrapping anarticle having a silver surface with said material.

12. The process in accordance with claim 11 wherein the wrappingmaterial is cotton cloth.

13. The process in accordance with claim 11 wherein the wrappingmaterial is paper.

14. A process for protecting articles having a silver surface whichcomprises impregnating a wrapping material with an aqueous solution ofzinc acetate, precipitating a zinc salt in said wrapping material byimpregnating said materiai with an aqueous solution of an alkali metalsalt of a member of the group consisting of carbonate and borate, in anamount in excess of that required to react with said zinc acetate andsuflicient to provide an alkaline condition in said wrapping materialwhen the same is moistened subsequent to its manufacture, drying saidwrapping material and wrapping an article having a silver surface withsaid material.

References Cited in the file of this patent UNITED STATES PATENTS Re.18,379 Herring Mar. 8, 1932 223,814 Pennington Jan. 27, 1880 380,515Pennington Apr. 3, 1888 1,203,241 Muntz Oct. 31, 1916 1,628,610 OsborneMay 10, 1927 1,949,781 Champlin Mar. 6, 1934 2,155,045 Griffith Apr. 18,1939 FOREIGN PATENTS 519,799 Great Britain Apr. 5, 1940

1. AN ARTICLE OF MANUFACTURE FOR PROTECTING SILVERWARE FROM TARNISH COMPRISING A FABRIC IMPREGNATED WITH A SALT OF ZINC, THE ANION OF WHICH IS A MEMBER OF THE GROUP CONSISTING OF A CARBONATE AND A BORATE AND CONTAINING SODIUM ACETATE, SAID IMPREGNATED FABRIC WHEN MOISTENED HAVING A PH FROM 7.0 TO 10.0. 